The applicants have developed a new animal model of HIV-1 disease characterized by a newly- derived pathogenic SHIV (SIV gag and HIV-1 env) that causes loss of CD4+ T-cells and development of AIDS in pigtailed macaques. Virus was selected by three serial transfers of infectious bone marrow to bone marrow of recipient animals. Using virus from infectious cerebrospinal fluid (CSF) for intra-vaginal inoculation, the applicants have shown that virus invades the systemic circulation across the vaginal mucosa, replicates in the lymphoid system causing loss of CD4+ T-cells, and invades the central nervous system (CNS). During the proposed period of grant support, the applicants will enhance the neurovirulence of this virus by more passages of the virus in macaques, using virus developing in the CSF of the recipient animals as intra-vaginal inoculum for the new animals. The final virus stock will be assayed for infectivity and disease in pigtailed macaques after intra-vaginal inoculation. Pathogenesis of the infection caused by virus will be assessed by measurement of viral DNA, infectious virus, and viral host-cell interactions in different tissues following inoculation of the animals. Studies at the molecular level will aim at identification of nucleotide sequences associated with the pathogenic virus. In addition, this agent will be used as homologous intravaginal challenge for vaccines. New challenge viruses are currently being constructed for future heterologous virus challenge experiments because of the relevance of this experiment to human vaccine development. In the immediate future, three vaccines given by the mucosal route will be evaluated for their ability to induce protection against infection and disease after challenge with homologous virus given intravaginally. The vaccines are: 1) fixed SHIV-infected pigtailed macaque lymphocytes inoculated into the pharyngeal tonsils and duodenum; 2) a macrophage tropic altered nef SIVmac used as a live virus vaccine; and 3) Salmonella expressing HIV env and SIV gag proteins. Protection from pathogenic SHIV will be correlated with mucosal and systemic immune responses. Infection in the animals will be assessed by tests for viral DNA and infectious virus in multiple sites. Disease will be assessed by loss of CD4+ T-cells, the onset of AIDS, and development of neurological disease.